1. Technical Field of the Invention
The present invention relates generally to the construction of diagrams representing the relationship between variable quantities; and, more particularly, to a method and apparatus for constructing diagrams representing the relationship between the bit error rate of a digital signal and one or more other variable quantities.
2. Description of Related Art
In communications systems, a number of factors can contribute to a degradation in the quality of a transmitted signal. For example, the quality of a signal can be degraded as a result of one or more of mechanisms for modulating the signal, transmitting the signal and detecting the signal. By evaluating signal degradation in a communications system, the performance of a particular system can be verified and information obtained from the evaluation can be utilized in the development of new systems.
Bit Error Rate Testers (BERTs) are often used to evaluate the performance of digital communications systems (see D. Derickson ed., Fiber Optic Test and Measurement, Hewlett-Packard Professional Books, Upper Saddle River, N.J., 07458, Prentice Hall PTR, 1998). A bit error rate tester measures bit error rate (BER) for a given setting of control variables associated with a Device Under Test (DUT). By plotting the BER versus different values of a particular control variable, a user is able to establish the margins of that variable for a given BER.
Exemplary measurement procedures result in diagrams referred to as xe2x80x9cV Curvesxe2x80x9d, xe2x80x9cBathtub Curvesxe2x80x9d and xe2x80x9cEye Diagramsxe2x80x9d. To construct a V Curve, the user varies a threshold voltage while keeping other variables constant. To construct a Bathtub Curve, the user varies the bit timing while keeping other variables constant. To construct an Eye Diagram, some representation of the BER is plotted versus a pair of control variables. For example, offset time can be plotted along the x-axis and threshold voltage can be plotted along the y-axis. The BER at the plotted points can then be represented in various ways such as by a color map, a contour plot or some three-dimensional representation.
Embodiments in accordance with invention provide a method and apparatus for constructing a diagram representing a relationship between variable quantities that enables the diagram to be accurately constructed with a limited number of measurement points and in a reduced period of time.
A method for constructing a diagram representing a relationship between variable quantities according to the present invention comprises providing at least one first measurement point at which a relationship between variable quantities is measured for initially constructing the diagram. Thereafter, at least one location is selected at which the relationship between the variable quantities is to be measured, the at least one location being selected as a function of the at least one first measurement point. The relationship between the variable quantities is then measured at the at least one location to provide at least one additional measurement point for further constructing the diagram.
It has been discovered that by intelligently selecting locations at which the relationship between variable quantities are to be measured, a diagram that accurately represents the relationship between the variable quantities can be efficiently constructed. Intelligent selection of the measurement points may, for example, permit a reduction in the number of measurement points required to accurately construct the diagram, and may permit the diagram to be constructed in less time than by conventional diagram construction procedures.
Furthermore, the invention provides embodiments with other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent from the description below with reference to the following drawings.